T-REx™ Cell Lines stably express the tetracycline repressor protein (Table 1). They save significant time and effort when using the T-REx™ System. The T-REx™ Cell Lines are functionally tested by transient transfection with the positive control vector pcDNA™4⁄TO⁄lacZ. T-REx™ Cell Lines exhibit extremely low basal expression levels in the repressed state and high expression upon induction with tetracycline (Figure 1).